a) Field of the Invention
This application is directed to a device for cleaning filters, and in particular, diesel particulate filters.
b) Description of Related Art
A diesel particulate filter removes organic and inorganic particulate matter (PM) from the exhaust gas stream of an engine. The organic particulate is a complex blend of carbon, hydrogen and oxygen, and is a result of incomplete combustion of the diesel fuel in the cylinder. The inorganic portion of the PM has its source in the additives in the lubrication oil or fuel, and material eroded from the engine surfaces. Under optimal circumstances, the organic PM will fully combust during filter regeneration and thus leave the filter as gaseous CO2 and H2O. The inorganic component, on the other hand, can not be converted to gaseous components, and is trapped in the filter as various oxides (called “ash”). To maintain acceptable performance, the ash must be periodically removed from the filter.
Some installations of diesel particulate filters (DPFs) are made on engines which have operating temperatures too low to properly regenerate the filter (i.e., oxidize the organic PM). In these cases, the filter can become clogged with PM and potentially reduces the performance of the engine. In addition, a filter with a high soot load has a higher chance of permanent damage through uncontrolled regeneration than one with low soot load. In these cases of insufficiently high operating temperature, regular removal of the soot may be required.
Prior approaches to filter cleaning, as discussed in this section, have various flaws and shortcomings. Some of these approaches are overly complex, others are ineffective at removing tightly bound particulate, and others can lead to high PM emissions during the cleaning process.
The following illustrates the prior approaches (e.g., devices and methods) and their disadvantages.                I. Cleaning the DPF While it is Off the Engine        
A simple way to clean a filter is with a compressed air hose. The hose is directed into the exit face of the filter, thus blowing the soot out of the wall in the reverse direction to which it was initially deposited (i.e., backwashing or back flushing). This method is imprecise, potentially dangerous (compressed air hazards), requires the full attention of an operator, and if improperly performed can lead to emission of PM from the filter end as well as a poorly cleaned filter.
Heating the dirty filter in an oven to a high temperature can effectively remove the carbon-based particles, but requires significant energy input and does not remove the inorganic ash. After a heating cycle, the cool down period is significant, and the ash must be removed through vacuuming or washing.
A large blower can be used with the filter in the reverse flow orientation. Blowers of the appropriate size are expensive and noisy, and the technique may not remove the tightly bound material. Additionally, as the cleaning process progresses, the flow will preferentially go through the cleanest portion (i.e., areas with least pressure drop).
The ECS Unikat Combifilter (Engine Control Systems Europe AB, 2001) is a particulate filter and regeneration system which is designed to be installed on a vehicle or stationary engine. The first section of the device (the “regeneration section”) is an electrical heater which is used to regenerate the particulate filter (using electricity from the utility grid). With a change in flange configuration and removal of the filter section, the heater could be used as an off-engine regeneration device. When soot removal is required, the filter is removed from the vehicle and installed downstream of the Unikat Combifilter regeneration section. The device is started, and after the 2 to 8 hour regeneration period, the carbonaceous soot will be removed, but the ash remains within the filter.
The CombiClean system is also available from ECS, consists of the electrical regeneration device described above, a compressed air hose and nozzle, and an ash collection vacuum (ECS CombiClean brochure, #M21-0024 -December 2002). The system relies on manual application of compressed air to loosen the ash before the vacuuming process.
Many systems for cleaning industrial devices utilize a combination of liquid flow and ultrasound, which may be effective, but can be overly expensive In addition, the cleaning liquid can damage the catalytic coating or the matting material which secures the catalyst within its metal housing.
Others have described cleaning systems which involve backwashing with “cleaning fluid” until the filter is clean. However, many catalysts and their matting material are sensitive to large amounts of water or solvents. Solvents have the additional disadvantage of requiring disposal. In addition, the flow of cleaning fluid might not be controlled locally (i.e., a single fluid stream flows over the unit), so that some sections of the filter might not be cleaned as well as others.
Vacuuming methods are used in the catalyst industry to remove excess washcoat or to remove ash from a flow-through monolith. The vacuum technique has the same disadvantages as the blower method.                II. Cleaning the DPF While it is on the Engine        
Methods for collecting particulate using several particulate filters with valves to control the flow path are described in a number of patent documents. A combination of valve settings can start the back-flush of one of the filters (i.e., the direction of gas flow is reversed and flows to push the soot out of the filter).
Shimoda et al. discloses a method which ‘backwashes’ a DPF to remove the particulate and ash collected in the filter. The backwashing occurs while the device is on the vehicle, and an impact air valve is used to provide a pressure wave to dislodge the particulate matter.
The above “back-flush” methods have the disadvantage that the ash from the lubricating oil never leaves the filter system (what is back-flushed from one element flows into another element), and manual cleaning will still be required.
Another system to clean the filter while it is on the engine is from Peugeot. The catalyst/filter system has a built-in ash/additive collection receptacle below the filter which allows the debris shaken loose by engine vibration to collect for later disposal. A method for washing with an air or water blower (while the engine is not running) is also disclosed.
The RC-2 Cleaning system (Environmental Solutions Worldwide Inc., Model RC-2 Cleaning System) is a device which can be attached to the inlet of the catalytic unit while it is on the vehicle, thus removing the need to wait for the device to cool to handling temperature. It is designed for flow through catalysts, which are relatively easy to clean, and thus does not provide a localized high pressure flow at the face of the catalyst.
Reversing the filter on the vehicle so that the exhaust gas passes from the clean side of the filter to the dirty side (thus expelling the particulate) can lead to a cleaner filter, but results in an unacceptable quantity of toxic particulate entering the air. In confined work areas such as maintenance garages, the likelihood of exposure to the particulate is high.                III. Rotating Arms        
Wade describes a device for regenerating a particulate trap using a rotating electrical heating element. A portion of the exhaust gas bleeds through the rotating arm and flows over the heating element. The combination of low flow rate and high temperature improves the chance of regeneration.
Williams discloses a dust collector with on-board programmable cleaning control. A rotating arm with a plurality of nozzles mounted upon it provides the back-flushing flow, thus causing the particulate to be removed from the bag surface and settle into a collection chamber. The control system operates the arm and nozzles to produce jets of cleaning fluid above the various bag units. The arm also contains a sensor for determining the dirtiness of each filter element (a pitot tube is suggested). The system described in the patent has several design elements which make it unsuitable for use in diesel particulate filter applications. First, DPFs are much smaller than dust collectors, and the nozzle designs in the above dust collector are specialized for large filters. A typical DPF is between 15 cm and 32 cm in diameter. The dust collector shown in the patent appears to be many meters in diameter. Second, DPFs can have many thousand cells, and thus focusing air on each individual cell is impractical. Other similar designs for dust collectors have the same shortcomings.